Electrode for lithium secondary battery and lithium secondary battery

ABSTRACT

An electrode for lithium secondary battery having a current collector and, deposited thereon, a thin film comprising silicon as a main component, characterized in that the thin film comprising silicon contains at least one of the elements belonging to the groups IIIa, Iva, Va, VIa, VIIa, VIII, Ib and IIb in the fourth, fifth and sixth periods of the Periodic Table (exclusive of copper (Cu)) at least in the surface portion thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrode for lithium secondarybattery and lithium secondary battery using this.

2. Related Art

In recent years, development in lithium secondary batteries has beenconducted enthusiastically. As for the lithium secondary battery, itsbattery characteristics such as charge-discharge voltage,charge-discharge cycle life characteristic and storage characteristicare greatly influenced by electrode active materials to be used.

In the electrode active materials capable of lithium storage andrelease, silicon is variously examined, since silicon can store lithiumby being alloyed with lithium so that its theoretical capacity is large.However, since silicon is alloyed with lithium to store, volumeexpansion and shrinkage during charge-discharge reaction are large. Forthis reason, the active material is pulverized and separated from acurrent collector, so that the charge-discharge cycle characteristicsare not good and thus silicon is not put into practical use.

In order to solve the above problem, silicon as the active material isattempted to be improved by doping impurity into silicon (JapanesePatent Laid-Open No. 10-199524 (1998)), and by using alloy powder ofsilicon and a different element (Japanese Patent Laid-Open No.2000-243389), but sufficient results are not yet obtained.

In addition, there suggests a method of using an intermetallic compoundof an element such as silicon and a metal or metalloid as a negativeelectrode active material so as to improve the cycle characteristic(Japanese Patent Laid-Open No. 10-223221 (1998)). However, a capacityretention rate is improved by making silicon into an intermetalliccompound, but the discharge capacity after cycles is not substantiallyimproved. The cause of this is such that since specified stoichiometryexists in an intermetallic compound, a quantity of element for absorbingand discharging lithium in unit volume becomes less, and an initialdischarge capacity is reduced more greatly than the case where theelement is used in element unit. For example, in the case of an Si₃Mstructure (M: metal or metalloid), a concentration of Si atom forabsorbing and discharging lithium is about 75 atomic % of the case of anSi element, and the concentration is reduced to about 67 atomic % in thecase of an Si₂M structure.

Meanwhile, as an electrode which can solve these problems, theapplicants of this invention find an electrode which is formed bydepositing a silicon thin film on a current collector according to athin film forming method such as a CVD method or a sputtering method. Itis confirmed that such a kind of the electrode shows highcharge-discharge capacity and excellent charge-discharge cyclecharacteristics.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electrode forlithium secondary battery, which is formed by depositing a silicon thinfilm on a current collector, and a lithium secondary battery using theelectrode which are capable of improving charge-discharge cyclecharacteristics.

The present invention is an electrode for lithium secondary batteryformed by depositing a thin film having silicon as a main component on acurrent collector, characterized in that the thin film having silicon asa main component contains at least one of elements belonging to groupsIIIa, IVa, Va, VIa, VIIa, VIII, Ib and IIb in fourth, fifth and sixthperiods of Periodic Table (exclusive of copper (Cu)) at least in asurface portion thereof.

Examples of the above element is concretely scandium (Sc), titanium(Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt(Co), nickel (Ni), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb),molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh),palladium (Pd), silver (Ag), cadmium (Cd), lanthanide series element,hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os),iridium (Ir), platinum (Pt), gold (Au) and hydrargyrum (Hg).

Examples of the above lanthanide series element are lanthanum (La),cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm),samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium(Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) andlutetium (Lu).

In the above elements, elements belonging to particularly groups VIII,Ib and IIb are preferable because their diffusion factor in silicon ishigh. More concretely, these elements are iron, cobalt, nickel, zinc,ruthenium, rhodium, palladium, silver, cadmium, osmium, iridium,platinum, gold and hydrargyrum.

It is preferable that at least one element selected particularly fromcobalt, zinc, iron, zirconium, nickel, silver and manganese is used. Itis preferable that in them, at least one electrode selected particularlyfrom cobalt, zinc, iron, zirconium and nickel is used.

In the present invention, at least the surface of the thin filmcomprising silicon as a main component contains the above elements. Itis considered that reaction between the thin film surface and theelectrolyte can be suppressed by containing the elements in the surfaceportion.

In addition, the entire thin film may contain the above elements. It isconsidered that expansion and contraction of the thin film due tocharge-discharge reaction can be reduced by containing the elements inthe thin film. Moreover, it is considered that mechanical properties ofthe thin film are changed by including the above elements in the thinfilm and the pulverization of the thin film can be suppressed. Further,since the expansion and contraction of the thin film can be reduced, astress which is exerted upon the current collector from the thin film atthe time of charge and discharge can be reduced, so that it isconsidered that generation of wrinkles on the current collector can besuppressed. The volume capacity density at the time of assembling abattery can be improved by suppressing the generation of wrinkles on thecurrent collector.

In addition, densifying of the thin film occurs by including the aboveelements in the thin film. For this reason, reduction of percentage ofan amount of the active material elements per unit volume is suppressedeven if the above elements are contained in the thin film. Moreover, inthe case where an amount of the elements to be contained is within aspecified range, an amount of the active material element per unitvolume is maintained equivalently or increased. At this time, adischarge capacity per unit volume can obtain the equivalent or largervalue.

In the present invention, it is preferable that the above element andsilicon form a solid solution in the thin film. For example in the casewhere the element is cobalt, it is preferable that not an intermetalliccompound of silicon and cobalt but the solid solution of silicon andcobalt is formed and the cobalt is contained in the state of the solidsolution.

In addition, it is preferable that the solid solution is in anon-equilibrium state. In the equilibrium state, only germanium whichforms the solid solution together with silicon is known, and the solidsolution of the above element and silicon exists only in thenon-equilibrium state.

For example in the case where the element is cobalt, according to abinary state diagram of silicon and cobalt, silicon and cobalt formvarious kinds of intermetallic compounds in a wide rage of abundanceratio. However, the solid solution is not formed in the wide range ofabundance ratio, and only a possibility that the solid solution isformed only in a range where a slight amount of any one of them iscontained is discovered. Here, the intermetallic compound is a compoundwhich has a specified crystal structure in which metals are combinedwith a specified ratio. Since the binary state diagram is based on theequilibrium state, a judgment cannot be made by the binary state diagramas to whether the non-equilibrium solid solution is formed. It isconsidered that since the solid solution is the non-equilibrium solidsolution, the thin film structure is not broken even by charge-dischargereaction and lithium can be absorbed and released.

It is preferable that a content of the above element in the thin film isnot more than 30 weight %, and more preferably not more than 20 weight%. When the content of the element in the thin film becomes too large,this is not preferable because the charge-discharge capacity of the thinfilm is lowered. Moreover, it is preferable that the content of theelement in the thin film is not less than 0.1 weight %, and morepreferably not less than 1 weight %. When the content of the element istoo small, an effect of the invention which suppresses a reaction withan electrolyte and improves the charge-discharge cycle characteristicscannot be occasionally obtained sufficiently. Therefore, it ispreferable that the content of the element is 0.1 to 30 weight %, andmore preferably 1 to 20 weight %.

It is preferable that the content of the element in the thin film is notmore than 17 atomic %. The reason for this is not clear, but it isconsidered that when the content of the element in the thin film is toolarge, aggregation of the element easily occurs, and this easily causesthe pulverization of the active material layer, and thus the effect forimproving the cycle characteristics is reduced.

It is preferable that the thin film of the present invention isseparated into columns by gaps formed in its thickness direction asillustrated in FIG. 3. Since gaps exist around the columnar portions,and the gaps absorb a stress generated by expansion and contraction ofthe thin film during the charge-discharge cycles, and the generation ofa stress for falling off the thin film from the current collector can besuppressed. Therefore, an adhesion state of a bottom portion of thecolumnar portion and the current collector can be maintainedsatisfactorily.

In addition, it is preferable that at least a not less than half portionof the thickness of the thin film is separated into the columns by thegaps in the thickness direction of the thin film.

Further, in the case where an uneven portion is formed on the surface ofthe thin film and the gaps, in which a recessed portion of the unevenportion is its end, is formed on the thin film, the gaps may be formedso that the columnar portion includes at least one convex portion on thethin film surface. In this case, the gaps may be formed so that thecolumnar portion includes a plurality of convex portions.

The gaps formed on the thin film may be formed by charge and dischargeafter initial charge and discharge. In this case, for example, theuneven portion is formed on the surface of the thin film before chargeand discharge, and the gaps, in which a recessed portion of the unevenportion on the surface of the thin film is its end, is formed by chargeand discharge after initial charge and discharge, and these gaps mayseparate the thin film into the columns.

The uneven portion on the surface of the thin film may be formedcorrespondingly to an uneven portion on the surface of the currentcollector as a ground layer. Namely, the current collector having theuneven portion on its surface is used and the thin film is formedthereon, so that the uneven portion can be provided on the surface ofthe thin film.

It is preferable that surface roughness Ra of the current collector isnot less than 0.01 μm, more preferably 0.01 to 1 μm, and more preferably0.05 to 0.5 μm. The surface roughness Ra is determined by JapaneseIndustrial Standard (JIS B 0601-1994), and it can be measured by, forexample, a surface roughness meter.

In the present invention, it is preferable that the surface roughness Raof the current collector has a relationship of Ra≦t with respect to athickness t of the active material thin film. Moreover, it is preferablethat the surface roughness Ra of the current collector and the meanspacing S of local peaks of profile satisfy the relationship of 100Ra≧S.The mean spacing S of local peaks of profile is determined by theJapanese Industrial Standard (JIS B 0601-1994), and it can be measuredby, for example, a surface roughness meter.

The shape of the convex portion of the uneven portion on the surface ofthe current collector is not particularly limited, but is preferablycone shape, for example.

In addition, it is preferable that an upper portion of the columnarportion has a round shape in order to avoid concentration of an electriccurrent due to the charge-discharge reaction.

In the present invention, the gaps in the thickness direction formed onthe thin film may be formed by charge and discharge after initial chargeand discharge, or previously formed before charge and discharge. As amethod of previously forming such gaps on the thin film before chargeand discharge, a method or the like of allowing the thin film of theelectrode to absorb and release lithium or the like before assembling abattery is used, so that the volume of the thin film is expanded andcontracted and the gaps can be formed. Needless to say, in the casewhere an active material which does not contain lithium is used as thepositive electrode, a battery may be assembled in a state that the thinfilm absorbs lithium. Moreover, a resist film or the like which ispatterned by photolithography is used and the thin film is formed intocolumns so that the thin film which is separated into columns by thegaps may be obtained.

Generally silicon is roughly classified into amorphous silicon,microcrystalline silicon, polycrystalline silicon and single crystallinesilicon according to a difference in crystallinity. A peak of theamorphous silicon in the vicinity of 520 cm⁻¹ corresponding to thecrystalline region in the Raman spectroscopy analysis is notsubstantially detected. As for the microcrystalline silicon, both a peakin the vicinity of 520 cm⁻¹ corresponding to the crystalline region anda peak in the vicinity of 480 cm⁻¹ corresponding to the amorphous regionare substantially detected by the Raman spectroscopy analysis.Therefore, the microcrystalline silicon is substantially structured bythe crystalline region and the amorphous region. As for thepolycrystalline silicon and single crystalline silicon, their peaks inthe vicinity of 480 cm⁻¹ corresponding to the amorphous region are notsubstantially detected by the Raman spectroscopy analysis.

In the present invention, a microcrystalline silicon thin film and anamorphous silicon thin film are preferable as the silicon thin filmcontaining the above element.

Further, as the thin film containing silicon of the present invention,besides the above silicon thin films, a silicon-germanium alloy thinfilm can be used. As the silicon-germanium alloy thin film, amicrocrystalline silicon-germanium alloy thin film and an amorphoussilicon germanium thin film are preferably used. Microcrystalline andamorphous of the silicon-germanium alloy thin film can be determinedsimilarly to the above silicon thin films. Since silicon and germaniumcan be mixed with each other to produce a uniform solid solution andeach of them provides good results in the present invention, it isconsidered that silicon-germanium alloy which is alloy of them alsoprovides good results.

In the present invention, the method of forming the thin film on thecurrent collector is not particularly limited, but for example, a CVDmethod, a sputtering method, a vacuum evaporation method, a sprayingmethod or a plating method can be used. In these thin film formingmethods, the CVD method, the sputtering method and the vacuumevaporation method are preferably used.

A method of containing the above element in the thin film, for examplein the case of the CVD method, a method of mixing a source gascontaining the above element with a source gas of silicon and dissolvingthe mixed gas so as to form the thin film can be used. Moreover, in thecase of the sputtering method, a method of arranging a target of siliconand a target of the above element so as to form the thin film can beused. In the case of the vacuum evaporation method, a method ofarranging a vacuum evaporation source of silicon and a vacuumevaporation source of the above element so as to form the thin film canbe used.

The current collector used in the present invention is not particularlylimited as long as the thin film can be formed on it with satisfactoryadhesion. As a concrete example of the current collector, at least oneof the current collectors selected from copper, nickel, stainless steel,molybdenum, tungsten and tantalum can be used.

It is preferable that the current collector has a thin thickness and ismade of a metal foil. It is preferable that the current collector isformed by a material which is not alloyed with lithium, and aparticularly preferable material is copper. It is preferable that thecurrent collector is made of a copper foil whose surface is roughed.Such copper foil is electrolytic copper foil. The electrolytic copperfoil is obtained, for example, in such a manner that a metallic drum isdipped in an electrolyte in which copper ion is dissolved and while thedrum is being rotated, an electric current is allowed to flow so thatcopper is separated out on the surface of the drum and is peeled. Onesurface or both surfaces of the electrolytic copper foil may be subjectto a roughing treatment or a surface treatment.

In addition, copper is deposited on a surface of rolled copper foil bythe electrolytic method so that copper foil with roughed surface may beobtained.

Further, an intermediate layer is formed on the current collector andthe thin film may be formed on the intermediate layer. In this case, itis preferable that the intermediate layer contains a component which iseasily diffused in the thin film, and for example, a copper layer ispreferable. For example, the current collector in which the copper layeris formed on a nickel foil (electrolytic nickel foil or the like) with aroughed surface may be used. Moreover, a nickel foil which is roughed bydepositing copper on the nickel foil using the electrolytic method maybe used.

The gaps formed on the thin film in the present invention may be formedalong low-density regions previously formed to extend in the thicknessdirection in the thin film. Such low-density regions are formed, forexample, so as to extend upward from the recessed portion of the unevenportion on the surface of the current collector.

In the present invention, it is preferable that a component of thecurrent collector is diffused in the thin film. Such diffusion of thecurrent collector component into the thin film makes it possible toheighten the adhesion between the current collector and the thin film.Moreover, in the case where an element such as copper which is notalloyed with lithium is diffused as the current collector component,since alloying with lithium is suppressed in the diffused region,expansion and contraction of the thin film due to charge-dischargereaction can be suppressed, so that generation of stress which causesfalling-off of the active material thin film from the current collectorcan be suppressed.

In addition, it is preferable that the concentration of the currentcollector component diffused in the thin film is high in the vicinity ofthe current collector, and the concentration is reduced as getting nearto the surface of the thin film. When the thin film has such aconcentration gradient of the current collector component, suppressionof expansion and contraction due to the charge-discharge reaction exertsupon the vicinity of the current collector more strongly, so thatgeneration of the stress which causes falling-off of the active materialthin film in the vicinity of the current collector can be easilysuppressed. Moreover, the concentration of the current collectorcomponent is reduced as getting near to the thin film surface, so thathigh charge-discharge capacity can be maintained.

Further, it is preferable the diffused current collector component doesnot form an intermetallic compound with the thin film component in thethin film but forms a solid solution. Here, the intermetallic compoundhas a specified crystal structure in which metals are combined with aspecified ratio. When the thin film component and the current collectorcomponent do not form the intermetallic compound but form the solidsolution in the thin film, the adhesion between the thin film and thecurrent collector becomes satisfactory, so that higher charge-dischargecapacity can be obtained.

An impurity other than the above element may be doped in the thin filmof the present invention. Examples of such an impurity are elements suchas phosphorus, aluminum, arsenic, antimony, boron, gallium, indium,oxygen and nitrogen.

In addition, the thin film of the present invention may be formed bylaminating a plurality of layers. In the respective laminate layers,composition, crystallinity, concentration and the like of the elementand impurity may differ. Moreover, the thin film may have a gradientstructure in the thickness direction. For example, the gradientstructure can be such that composition, crystallinity, concentration andthe like of the element and impurity are changed in the thicknessdirection.

In addition, lithium may be previously absorbed by or added into thethin film of the present invention. Lithium may be added when the thinfilm is formed. Namely, the thin film containing lithium is formed, sothat lithium may be added to the thin film. Moreover, after the thinfilm is formed, lithium may be absorbed by or added to the thin film. Asa method of allowing lithium to absorb or be added, a method of allowinglithium to absorb or be added electrochemically can be used.

Further, the thickness of the thin film of the present invention is notparticularly limited, but the thickness can be, for example, not morethan 20 μm. Moreover, in order to obtain high charge-discharge capacity,it is preferable that the thickness is not less than 1 μm.

A lithium secondary battery of the present invention is characterized byincluding a negative electrode composed of the above electrode of thepresent invention, a positive electrode and a nonaqueous electrolyte.

An electrolyte solvent used for the lithium secondary battery of thepresent invention is not particularly limited, but its example is amixed solvent of cyclic carbonate such as ethylene carbonate, propylenecarbonate, butylene carbonate or vinylene carbonate and chain carbonatesuch as dimethyl carbonate, methylethyl carbonate or diethyl carbonate.Moreover, an example of the electrolyte solvent is a mixed solvent ofthe above cyclic carbonate and an ether solvent such as1,2-dimethoxyethane or 1,2-diethoxyethane or a chain ester such asγ-butyrolactone, sulfolane or methyl acetate. Moreover, examples ofelectrolyte solute are LiPF₆, LiBF₄, LiCF₃SO₃, LiN (CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiN (CF₃SO₂) (C₄F₉SO₂), LiC (CF₃SO₂)₃, LiC (C₂F₅SO₂)₃,LiAsF₆, LiClO₄ , Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂ and the like and their mixture.Further, examples of electrolyte are a gel type polymer electrolyte, inwhich polymer electrolyte such as polyethylene oxide, polyacrylonitrileor polyvinylidene fluoride is impregnated into an electrolyte solution,and inorganic solid electrolyte such as LiI, Li₃N. The electrolyte ofthe lithium secondary battery of the present invention can be usedwithout restriction as long as a Li compound imparting an ionicconductivity and a solvent for dissolving and retaining the Li compoundare not decomposed at voltages during charge discharge and storage ofthe battery.

Examples of the positive electrode active material of the lithiumsecondary battery of the present invention are lithium-containingtransition metal oxides such as LiCoO₂, LiNiO₂, LiMn₂O₄, LiMnO₂,LiCo_(0.5)Ni_(0.5)O₂ or LiNi_(0.7)Co_(0.2)Mn_(0.1)O₂ and metal oxideswhich do not contain lithium such as MnO₂. Moreover, besides them, anymaterials where lithium is inserted into and separated fromelectrochemically can be used without restriction.

A lithium secondary battery of another aspect of the present inventionis characterized by including a positive electrode composed of the aboveelectrode of the present invention, a negative electrode and anonaqueous electrolyte.

As the nonaqueous electrolyte, similar one to the above nonaqueouselectrolyte can be used.

As the negative electrode, for example, lithium metal, bismuth-lithiumalloy, antimony-lithium alloy or the like can be used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a lithium secondary battery manufacturedin an embodiment of the present invention.

FIG. 2 is a cross sectional view showing a structure where electrodesare combined in the lithium secondary battery shown in FIG. 1.

FIG. 3 is a schematic sectional view showing a thin film of an electrodeof an embodiment of the present invention deposited on a currentcollector and divided into columns by gaps formed in its thicknessdirection.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

There will be further detailed below the present invention based onembodiments but the present invention is not limited to the followingembodiments and the invention is suitably modified so as to be carriedout without departing from the gist of the invention.

(Experiment 1)

[Manufacturing of Negative Electrode]

Electrolytic copper foil (thickness: 18 μm, surface roughness: Ra=0.188μm) was used as a current collector, and a thin film was formed on theelectrolytic copper foil by an RF sputtering method. As the thin film, asilicon-cobalt thin film, a silicon-chromium thin film and a siliconthin film were formed. As the silicon-cobalt thin film, five kinds ofthin films in which their cobalt content are different from one anotherwere formed.

The sputtering conditions were such that sputter gas (Ar) flow rate: 10sccm, a substrate temperature: room temperature (without heating),reactive pressure: 0.665 Pa (5×10⁻³ Torr) and a high frequency power:500 W. A single crystalline silicon target (diameter: 4 in (100 mm)) wasused as a target, and as for fabricating samples which contain cobalt asa different element, cobalt (Co) chips were arranged on the silicon (Si)target, and as for fabricating samples which contain chromium as adifferent element, chromium (Cr) chips were arranged on the silicontarget. The thin films were formed on the electrolytic copper foil of100 mm×100 mm so that their thickness became about 5 μm.

When the obtained thin films were subjected to Raman spectroscopyanalysis, a peak in the vicinity of 480 cm⁻¹ was detected but a peak inthe vicinity of 520 cm⁻¹ was not detected. As a result, it was foundthat the obtained thin films were thin films comprising amorphoussilicon as a main component. Moreover, as for the thin films containingcobalt or chromium, the content of respective elements was determined byX-ray fluorescence analysis. The content of different elements in therespective thin films and the arrangement state of the chips in thetarget at the time of sputtering are shown in Table 1.

TABLE 1 Different Element Type Content Arrangement State of Chip Co  1weight % Arrange one Co chip of 1 mm × 1 mm on Si target. Co  5 weight %Arrange one Co chip of 5 mm × 5 mm on Si target. Co 10 weight % Arrangeone Co chip of 10 mm × 10 mm on Si target. Co 20 weight % Arrange fourCo chips of 5 mm × 5 mm on Si target. Co 40 weight % Arrange four Cochips of 10 mm × 10 mm on Si target. Cr  5 weight % Arrange four Crchips of 10 mm × 10 mm on Si target.

When the content of Co in the thin films shown in Table 1 is convertedinto atomic %, 1 weight %, 5 weight %, 10 weight %, 20 weight % and 40weight % are converted into 0.5 atomic %, 2 atomic %, 5 atomic %, 11atomic % and 24 atomic %, respectively. Further, 5 weight % of Cr is 3atomic %.

The electrolytic copper foils on which the thin films were formed werecut out into 2.5 cm×2.5 cm, and they were dried at 100° C. for 2 hoursin a vacuum. An electrode which contained 1 weight % of cobalt was A1,an electrode which contained 5 weight % of cobalt was A2, an electrodewhich contained 10 weight % of cobalt was A3, an electrode whichcontained 20 weight % of cobalt was A4, an electrode which contained 40weight % of cobalt was A5, an electrode which contained 5 weight % ofchromium was B1, an electrode which was made of the amorphous siliconthin film uncontaining a different element was X1. These electrodes wereused as a negative electrode when the following batteries weremanufactured.

[Manufacturing of Positive Electrode]

85 weight % of LiCoO₂ powder having an average particle diameter of 10μm, 10 weight % of carbon powder as an electrically conductive agent and5 weight % of polyvinylidene fluoride powder as a binding agent weremixed, and N-methyl pyrolidone was added to the obtained mixture andkneaded so that slurry was manufactured. The slurry was applied to onesurface of the current collector made of aluminum foil with a thicknessof 20 μm by a doctor blade method. This current collector was dried at100° C. for two hours in a vacuum and was cut out into 2.0 cm×2.0 cm soas to be a positive electrode.

[Preparation of Electrolyte]

1 mol/l of LiPF₆ was dissolved in a solvent obtained by mixing ethylenecarbonate and diethyl carbonate with a volume ratio of 3:7 so thatelectrolyte was prepared.

[Manufacturing of Battery]

The above positive electrode and the above negative electrode werelaminated with a polyethylene fine porous film in between in a glove boxin an atmosphere of argon gas, and this was inserted into a case bodymade of an aluminum laminate material. 500 μl of the electrolyte waspoured into this case body so that a lithium secondary battery wasmanufactured. A design capacity of the battery is 14 mAh.

FIG. 1 is a plan view showing the manufactured lithium secondarybattery. As shown in FIG. 1, the positive electrode 1 and the negativeelectrode 3 are combined with a separator 2 made of a polyethylene fineporous film in between and they are inserted into the case body 4. Afterinserting into the case body 4, the electrolyte is poured and a sealingportion 4 a of the case body 4 is sealed so that the lithium secondarybattery is manufactured.

FIG. 2 is a cross sectional view showing a battery combined state in thebattery. As shown in FIG. 2, the positive electrode 1 and the negativeelectrode 3 are combined with the separator 2 in between so as to beopposed to each other. In the positive electrode 1, a positive electrodeactive material layer 1 a is provided onto a positive electrode currentcollector 1 b made of aluminum and the positive electrode activematerial layer 1 a comes in contact with the separator 2. Moreover, inthe negative electrode 3, a negative electrode active material layer 3 ais provided onto a negative electrode current collector 3 b made ofcopper and the negative electrode active material layer 3 a comes incontact with the separator 2.

As shown in FIG. 2, a positive electrode tab 1 c made of aluminum fortaking out is attached to the positive electrode current collector 1 b.Moreover, a negative electrode tab 3 c made of nickel for taking out isattached also to the negative electrode current collector 3 b.

[Measurement of Charge-discharge Cycle Characteristics]

The charge-discharge cycle characteristics of the above batteries wereevaluated. Charge was carried out up to 4.20 V by a constant current of14 mA, and constant voltage charge with cycle of 4.20 V was carried outup to 0.7 mA. The discharge was carried out up to 2.75 V by the constantcurrent of 14 mA, and this procedure was one cycle. The capacityretention rate after 90 cycles was obtained by the following equation.The result is shown in Table 2. Here, the measurement was conducted at25° C.Capacity retention rate (%)=(discharge capacity at 90th cycle/dischargecapacity at first cycle)×100

In addition, Table 2 shows the results of X-ray diffraction analysis(XRD: X-ray source: CuKα) of the thin films formed as the respectiveelectrodes.

TABLE 2 Discharge Discharge Capacity Capacity Different at 1st at 90thCapacity Element Cycle Cycle Retention Electrode Type Content (mAh)(mAh) Rate (%) XRD Result A1 Co  1 weight % 13.5 6.8 50 No Peak in Si—CoCompound A2 Co  5 weight % 13.6 9.9 73 No Peak in Si—Co Compound A3 Co10 weight % 13.7 11.8 86 No Peak in Si—Co Compound A4 Co 20 weight %13.6 11.4 84 No Peak in Si—Co Compound Peak Exists in A5 Co 40 weight %4.9 0 0 Si—Co Compound B1 Cr  5 weight % 13.4 5.6 42 No Peak in Si—CoCompound X1 — 0 13.4 3.5 26 —

As is clear from Table 2, in the batteries using the electrodes A1through A4 containing 1 to 20 weight % of cobalt and the battery usingthe electrode B1 containing 5 weight % of chromium, the capacityretention rate is higher than that of the battery using the electrode X1made of the amorphous silicon thin film uncontaining a differentelement, and thus it is found that the charge-discharge cyclecharacteristics are improved.

In the electrode A5, as the result of the X-ray diffraction analysis,the peak of the intermetallic compound of silicon and cobalt is found onthe thin film. On the contrary, on the electrodes A1 through A4, thepeak of intermetallic compound is not found, and thus it is found thatthe cobalt and the silicon form a solid solution in the thin film.Similarly in the electrode B1, it is found that the chromium and thesilicon form a solid solution in the thin film.

(Experiment 2)

Similarly to the experiment 1, thin films were formed on theelectrolytic copper foil by an RF sputtering method. As the thin film, asilicon-zinc thin film, a silicon-iron thin film, a silicon-nickel thinfilm, a silicon-zirconium thin film, a silicon-silver thin film, asilicon-manganese thin film, a silicon-molybdenum thin film, asilicon-tantalum thin film, a silicon-niobium thin film, asilicon-titanium thin film, a silicon-tungsten thin film and asilicon-vanadium thin film were formed.

As for a target, similarly to the experiment 1, the chips made of theabove different elements were arranged on the single crystalline silicontarget. The sizes and numbers of the chips are shown in Table 3.

The obtained thin films were subjected to the Raman spectroscopyanalysis, so that it was confirmed that they were thin films comprisingamorphous silicon as a main component.

Similarly to the experiment 1, the lithium secondary batteries weremanufactured by using the electrolytic copper foils formed with the thinfilms, and similarly to the experiment 1 their charge-discharge cyclecharacteristics were evaluated. The results are shown in Table 3. Table3 also shows the result of the lithium secondary battery using thecomparative electrode X1 in the experiment 1.

TABLE 3 Size (mm) Discharge Discharge Different and Capacity CapacityElement Number at 1st at 90th Capacity Content of Cycle Cycle RetentionElectrode Type (weight %) Chips (mAh) (mAh) Rate (%) XRD Result C Zn 3 8× 8 13.8 11.6 84 No Peak in (4 pieces) Si—Zn Compound D Fe 8 10 × 1014.0 12.2 87 No Peak in (1 piece) Si—Fe Compound E Ni 7 5 × 5 14.0 11.382 No Peak in (1 piece) Si—Ni Compound F Zr 12 10 × 10 13.6 11.1 84 NoPeak in (2 pieces) Si—Zr Compound G Ag 20 10 × 10 13.6 10.2 71 No Peakin (1 piece) Si—Ag Compound H Mn 10 10 × 10 13.6 8.8 65 No Peak in (1piece) Si—Mn Compound I Mo 20 10 × 10 11.9 5.8 49 No Peak in (2 pieces)Si—Mo Compound J Ta 13 10 × 10 10.6 4.4 37 No Peak in (2 pieces) Si—TaCompound K Nb 25 10 × 10 13.3 3.4 39 No Peak in (3 pieces) Si—NbCompound L Ti 5 10 × 10 14.0 2.3 27 No Peak in (2 pieces) Si—Ti CompoundM W 20 10 × 10 12.6 3.1 27 No Peak in (2 pieces) Si—W Compound N V 5 10× 10 11.8 2.9 27 No Peak in (1 piece) Si—V Compound X1 — — — 13.4 3.5 26—

As for the electrodes C through N shown in Table 3, a relationshipbetween weight % and atomic % is shown in Table 4.

TABLE 4 Content Electrode Type Weight % Atomic % C Zn 3 1 D Fe 8 4 E Ni7 4 F Zr 12 4 G Ag 20 6 H Mn 10 5 I Mo 20 7 J Ta 13 2 K Nb 25 9 L Ti 5 3M W 20 4 N V 5 3

As is clear from Table 3, the batteries, which use the electrodes Cthrough N using the amorphous silicon thin film containing differentelements according to the present invention, have higher capacityretention rate than that of the battery which uses the electrode X1 ofthe amorphous silicon thin film uncontaining different elements, andthus it is found that the charge-discharge cycle characteristics areimproved. In addition, as the result of the X-ray diffraction analysis,since the peak of the intermetallic compound of silicon and thedifferent elements is not found on the thin film, it is found that thedifferent elements and silicon form a solid solution in the thin films.

The electrodes G through H, which contain zinc, iron, nickel, zirconium,silver and manganese in the above different elements, show satisfactorycapacity retention rate, and in them particularly zinc, iron, zirconiumand nickel show satisfactory capacity retention rate of not less than80%.

(Experiment 3)

The electrolytic copper foil (thickness: 18 μm, surface roughness:Ra=0.188 μm) was used as the current collector, and thin films wereformed on the electrolytic copper foil by the sputtering method using DCpulse discharge. As the thin films, a silicon-cobalt thin film, asilicon-zinc thin film, a silicon-iron thin film and a silicon-zirconiumthin film were formed.

The sputtering conditions were such that a sputter gas (AR) flow rate:85 sccm, a substrate temperature: room temperature (without heating),reactive pressure: 0.532 Pa (4×10⁻³ Torr), a DC pulse frequency: 100kHz, a pulse width: 1.696 μs and an applying power: 1300 W. The thinfilms were formed on the electrolytic copper foil of 175 mm×400 mm sothat their thickness becomes about 6 μm.

As the target, silicon alloy targets of 35 cm×20 cm, which were obtainedby mixing respective elements including cobalt, zinc, iron and zirconiumwith silicon and sintering them, were used. The type and concentrationof the mixed elements in the respective targets and the concentration ofthe mixed elements in the obtained thin films are shown in Table 5.Here, the concentration of the elements in the thin films was measuredby X-ray fluorescence analysis.

In addition, the obtained thin films were subjected to the Ramanspectroscopy analysis, and it was confirmed that they were thin filmscomprising amorphous silicon as a main component.

The electrolytic copper foils on which the thin films were formed wereused so that the lithium secondary batteries were manufactured similarlyto the experiments 1 and 2, and the charge-discharge cyclecharacteristics were evaluated similarly to the experiments 1 and 2. Theresults are shown in Table 5. Here, as a comparison, a comparativeelectrode X2, which was formed with the amorphous silicon thin filmusing crystalline silicon as a target, was manufactured. The result ofthe lithium secondary battery using this comparative electrode X2 isalso shown in Table 5.

TABLE 5 Discharge Discharge Mixed Element Capacity CapacityConcentration Concentration Concentration at 1st 90th Capacity in Targetin Film in Film Cycle Cycle Retention XRD Electrode Type (weight %)(weight %) (atomic %) (mAh) (mAh) Rate (%) Result O Co 20 20 11 13.6 9.167 No Peak in Si—Co Compound P Co 30 30 17 12.5 9.0 72 No Peak in Si—CoCompound Q Zn 5 4 2 13.9 8.5 61 No Peak in Si—Zn Compound R Fe 10 10 513.4 10.3 77 No Peak in Si—Fe Compound S Zr 10 11 4 13.7 9.2 67 No Peakin Si—Zr Compound X2 — — — — 14.2 6.2 44 —

As is clear from Table 5, the batteries, which use the electrodes Othrough S using the amorphous silicon thin film containing the differentelements according to the present invention, have higher capacityretention rate than that of the battery which uses the electrode X2using the amorphous silicon thin film uncontaining the different elementwhich is formed under the same conditions, and thus it is found that thecharge-discharge cycle characteristics are improved. Moreover, as theresult of the X-ray diffraction analysis, a peak of the intermetalliccompounds of silicon and the different elements in the thin films wasnot found. Therefore, it is found that the different elements andsilicon form a solid solution in the thin films.

The electrode O was taken out at the time of end of the fourth cycle,its SEM observation was conducted. As a result, gaps which extends up toa valleylike portion as an end of an uneven portion on the thin filmsurface were formed in a thickness direction of the entire thin film,and it was confirmed that the thin film was separated into columns bythese gaps.

A weight of Si per unit area which is determined by the X-rayfluorescence analysis was normalized by a film thickness so that weightdensity of Si per unit volume was obtained. The weight density of theelectrode X2 was 2.22 g/cm³, whereas the weight density of the electrodeO was 2.13 g/cm³ although it contained 20 weight % (11 atomic %) ofcobalt. This shows that even if cobalt is added, lowering of the weightdensity and the atomic density of Si is suppressed.

Further, as discharge capacity density per unit volume is larger withinthe range in which the satisfactory cycle characteristic is obtained,the active material layer with smaller area and thinner thickness can beavailable. For this reason, this is an important value for design of thebatteries. Both the electrodes X2 and O had the discharge capacitydensity per unit volume of 6.8 Ah/cm³. Namely, although the electrode Ocontains 20 weight % (11 atomic %) of cobalt, it can have the same levelof the discharge capacity as that of the electrode X2. This andabove-described result show the denseness of the active material thinfilm are improved in the electrode O.

In addition, the discharge capacity density of the electrode P per unitvolume was 6.3 Ah/cm³ and was slightly lowered in comparison with theelectrode X2. However, the electrode P contains 30 weight % (17 atomic%) of cobalt, and when this is taken into consideration, the high valueis obtained. Moreover, the capacity retention rate becomes higher,thereby improving the cycle characteristics greatly.

INDUSTRIAL APPLICABILITY

According to the present invention, an electrode for lithium secondarybattery having high discharge capacity and excellent charge-dischargecycle characteristics can be obtained.

1. An electrode for lithium secondary battery formed by depositing athin film comprising silicon as a main component on a current collector,characterized in that said thin film comprising silicon as a maincomponent contains at least one of the elements (exclusive of copper(Cu)) belonging to groups IIIa, IVa, Va, VIa, VIIa, VIII, Ib and IIb infourth, fifth and sixth periods of Periodic Table at least in a surfaceportion thereof, and said thin film is divided into columns by gapsformed therein and extending in a thickness direction of said film froma surface of said film toward said current collector and said columnarportions are at their bottoms adhered to said current collector.
 2. Theelectrode for lithium secondary battery according to claim 1,characterized in that said entire thin film contains said element. 3.The electrode for lithium secondary battery according to claim 1,characterized in that said element is contained in said thin film so asto form a solid solution with silicon.
 4. The electrode for lithiumsecondary battery according to claim 3, characterized in that said solidsolution is a non-equilibrium solid solution.
 5. The electrode forlithium secondary battery according to claim 1, characterized in that acontent of said element in said thin film is not more than 30 weight %.6. The electrode for lithium secondary battery according to claim 1,characterized in that a content of said element in said thin film is notmore than 17 atomic %.
 7. The electrode for lithium secondary batteryaccording to claim 1, characterized in that said thin film is formed bya CVD, sputtering, vacuum evaporation, spraying, or plating process. 8.The electrode for lithium secondary battery according to claim 1,characterized in that said thin film is an amorphous silicon thin filmor a microcrystalline silicon thin film.
 9. The electrode for lithiumsecondary battery according to claim 1, characterized in that atomicdensity of silicon per unit volume in said thin film is equivalent orincreased by including said element in said thin film.
 10. The electrodefor lithium secondary battery according to claim 1, characterized inthat a discharge capacity in said thin film per unit volume isequivalent or increased by including said element in said thin film. 11.The electrode for lithium secondary battery according to claim 1,characterized in that said element is cobalt or chromium.
 12. Theelectrode for lithium secondary battery according to claim 1,characterized in that said element is at least one element selected fromzinc, iron, zirconium and nickel.
 13. The electrode for lithiumsecondary battery according to claim 1, characterized in that said gapsare formed along low-density regions which extend in the thicknessdirection of said thin film.
 14. The electrode for lithium secondarybattery according to claim 1, characterized in that said currentcollector is at least one current collector selected from copper,nickel, stainless steel, molybdenum, tungsten and tantalum.
 15. Theelectrode for lithium secondary battery according to claim 1,characterized in that surface roughness Ra of said current collector is0.01 to 1 μm.
 16. The electrode for lithium secondary battery accordingto claim 1, characterized in that said current collector is a copperfoil.
 17. The electrode for lithium secondary battery according to claim16, characterized in that both surfaces of said copper foil are roughed.18. The electrode for lithium secondary battery according to claim 16,characterized in that said copper foil is an electrolytic copper foil.19. The electrode for lithium secondary battery according to claim 1,characterized in that a component of said current collector is diffusedin said thin film.
 20. The electrode for lithium secondary batteryaccording to claim 19, characterized in that the component of saidcurrent collector is diffused with concentration distribution in whichthe concentration becomes high in the vicinity of said current collectorand the concentration becomes lower as further separating from saidcurrent collector.
 21. A lithium secondary battery, characterized bycomprising a negative electrode made of the electrode according to claim1, a positive electrode and nonaqueous electrolyte.
 22. A lithiumsecondary battery, characterized by comprising a positive electrode madeof the electrode according to claim 1, a negative electrode andnonaqueous electrolyte.